1. Field of the Invention
The present invention relates to a method of monitoring a laser crystallization (LC) process, and more particularly, to a method of on-line monitoring the result and the uniformity of the laser crystallization process quickly by utilizing a variable wavelength ellipsometry.
2. Description of the Prior Art
Nowadays, a liquid crystal display(LCD)is the most mature flat panel display technique. The applications for a liquid crystal display are extensive, such as mobile phones, digital cameras, video cameras, notebooks, and monitors. Due to the high quality vision requirements and the expansioninto new application fields, the LCD has developed toward high quality, high resolution, high brightness, and low price. A low temperature polysilicon thin film transistor (LTPS TFT), having a character of being actively driven, is a break-through in achieving the above objectives. Furthermore, a metal-oxide-semiconductor and the low temperature polysilicon thin film transistor in this technique are integrated in a same manufacturing process to fabricate a system on panel (SOP). The low temperature polysilicon thin film transistor therefore has become a booming development area for all of the vendors.
During the manufacturing process of the low temperature polysilicon thin film transistor liquid crystal display, a glass substrate tends to deform if the polysilicon film is directly formed at a high temperature since the resistance of the glass substrate to heat is merely up to 600xc2x0 C. As a result, an expensive quartz is utilized as the substrate for the traditional polysilicon thin film transistor liquid crystal display. The application is therefore limited to small sized liquid crystal display panels. Nowadays, a method to re-crystallize the amorphous silicon thin film has come with the tide of fashion and has become main stream. More particularly, the excimer laser annealing (ELA) process is most significant.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a method of forming a polysilicon thin film by utilizing an excimer laser annealing process. As shown in FIG. 1, an amorphous silicon thin film 12 having a thickness of approximate 500 xc3x85 is deposited on a glass substrate 10 first. Then the glass substrate 10 is disposed in a hermetic chamber (not shown) to perform the excimer laser annealing process. The method for depositing the amorphous silicon thin film 12 comprises a low-pressure chemical vapor deposition (LPCVD) process, a plasma enhanced chemical vapor deposition (PECVD) process, or a sputtering process. When performing the excimer laser annealing process, the amorphous silicon thin film 12 on the surface of the glass substrate 10 is irradiated by the laser pulse 14 of the excimer laser through a transparent window (not shown) on the upper surface of the chamber (not shown). The laser pulse 14 scans the regions within a process scope, which is determined previously, step-by-step to heat the amorphous silicon thin film 12 within the process scope rapidly. The amorphous silicon thin film 12 is therefore re-crystallized into a polysilicon thin film (not shown).
Moreover, the amorphous silicon thin film is melted and re-crystallized rapidly through absorption of the deep ultraviolet light during the excimer laser annealing process to form the polysilicon thin film. Such a quick absorption due to the short laser pulse only affects the surface of the amorphous silicon thin film and will not affect the glass substrate. Hence, the glass substrate is kept in a low temperature state. The excimer lasers frequently used comprise a XeCl laser, an ArF laser, a KrF laser, and a XeF laser. Since the different molecules will generate light with different wavelengths, the out energy density is therefore adjusted according to the thickness of the amorphous silicon thin film. For example, the output energy density is approximately 200 to 400 mJ/cm2 for an amorphous silicon thin film with a thickness of 500 xc3x85. After performing the excimer laser annealing process, the subsequent processes for forming the liquid crystal display panel are performed. The polysilicon thin film is used as a channel or a source/drain to form the driving circuit or the logic circuit on the liquid crystal display panel.
Since the quality of the amorphous silicon thin film 12 is a determinative factor for the characteristics of the subsequently formed polysilicon thin film, all of the parameters during the amorphous silicon thin film deposition process need to be strictly controlled. The amorphous silicon thin film with low hydrogen content, high thickness uniformity and low surface roughness is thus formed. In addition, many variables during the crystallization process, such as the magnitude of the laser energy density, the spatial uniformity of the laser energy, the overlapping ratio of the laser pulse, the substrate temperature during the laser annealing process and the atmosphere, will directly affect the grain size and the grain distribution after the crystallization process is completed. When non-uniform phenomenon occurs during the crystallization process, a strip type defect emerges.
In view of this, an inspection process is usually performed to monitor the result and the uniformity of the laser crystallization process. The method utilized in the inspection process usually comprises a visual inspection method, a scanning electron microscope (SEM) observation method, or a spreading resistance measurement (SR measurement) method.
However, all of the prior art methods of monitoring the laser crystallization process have drawbacks. The visual inspection method can""t provide an objective result. Moreover, when the dimensions of the substrate become larger and larger, the visual inspection method is not applicable. The method utilizing the scanning electron microscope to observe the grain microstructure is a destructive inspection method. Both the sample preparation and the sample observation take lots of time. Therefore, this method is not timely at all. In addition, the crystallization uniformity can""t be judged definitely by this method. The spreading resistance measurement method is also a destructive inspection method. In order to increase the film conductivity, dopants need to be doped into the sample and activated. The measurement result is readily affected by the doping uniformity and the activating uniformity.
Therefore, it is very important to develop a method of monitoring the laser crystallization process. This method should not only be a non-destructive inspection method, but is also applicable to a substrate of any size. Furthermore, this method should reflect the result and the uniformity of the crystallization process rapidly and definitely in the on-line circumstance.
It is thereforean object of the claimed invention to provide a method of monitoring the laser crystallization (LC) process, especially a method of on-line monitoring the result and the uniformity of the laser crystallization process quickly by utilizing a variable wavelength ellipsometry.
According to the claimed invention, a substrate is provided first. Then an amorphous silicon thin film is formed on a surface of the substrate. A laser crystallization process is thereafter performed to re-crystallize the amorphous silicon thin film into a polysilicon thin film by irradiating the amorphous silicon thin film with a laser pulse along a first direction. The laser pulse has an irradiation interval. After that, a light source provided by an optical instrument is focused into a micro spot with a diameter. The polysilicon thin film is irradiated by the micro spot along the first direction to obtain at least one measured spectrum. The micro spot has a moving distance relative to the substrate. Finally, a comparison step is performed to compare each measured spectrum with a pre-set spectrum. The diameter of the micro spot is smaller than the irradiation interval, the moving distance of the micro spot relative to the substrate is smaller than the irradiation interval.
The method of monitoring the laser crystallization process is to utilize a variable wavelength ellipsometry, which is employed in the production line, to focus the light source into a micro spot having a diameter smaller than the irradiation interval. The moving distance of the micro spot relative to the substrate is also smaller than the irradiation interval too. A non-destructive inspection is thus performed to the polysilicon film. As a result, the present invention method effectively avoids the not reflected strip type defect problem due to one of the irradiation intervals of the laser pulse being unsampled. The present invention method is suitable and convenient for on-line application. In addition, the present invention method is applicable to the substrate of any size to rapidly and definitely reflect the result and the uniformity of the crystallization process.